The present invention relates to a power supply device such as a series regulator or a constant-voltage power supply, and to a semiconductor integrated circuit device constituting such a power supply device.
FIG. 6 shows a circuit diagram showing the internal configuration of a conventionally used power supply device. This conventional power supply device is composed of switches 1 and 2, constant-current sources 3, 4, and 5 and a resistor R1 to which a supply voltage Vcc is applied through the switch 2, pnp-type transistors Tr1, Tr2, Tr3, Tr6, and Tr8, npn-type transistors Tr4, Tr5, and Tr7, an output terminal 6, and resistors R2 and R3 for dividing the output voltage appearing at the output terminal 6.
The transistor Tr1 has its base connected to the switch 1, has its emitter connected to the constant-current source 3, and has its collector grounded. The transistors Tr2 and Tr3 have their emitters connected to the constant-current source 4, have their bases connected respectively to the emitters of the transistors Tr1 and Tr6, and have their collectors connected respectively to the collectors of the transistors Tr4 and Tr5. The transistors Tr4 and Tr5 have their emitters grounded, and have their bases connected together. The transistor Tr4 has its collector connected to its base, and the transistor Tr5 has its collector connected to the base of the transistor Tr7.
The transistor Tr6 has its emitter connected to the constant-current source 5, has its base connected to the node between the resistors R2 and R3, and has its collector grounded. The transistor Tr7 has its collector connected to the resistor R1, and has its emitter grounded. The transistor Tr8 receives at its emitter the supply voltage Vcc through the switch 2, has its base connected to the resistor R1, and has its collector connected to the output terminal 6. The resistor R2 is connected to the output terminal 6, and the resistor R3 is grounded. When the switch 1 is switched to its contact xe2x80x9ca,xe2x80x9d the base of the transistor Tr1 is grounded, and, when the switch 1 is switched to its contact xe2x80x9cb,xe2x80x9d a voltage VBG is applied to the base of the transistor Tr1. Furthermore, to the output terminal 6, a capacitor Co is connected that provides phase compensation capacitance. The capacitor Co is grounded at the other end.
In this power supply device configured as described above, the constant-current sources 3, 4, and 5 and the transistors Tr1, Tr2, Tr3, Tr4, Tr5, and Tr6 together constitute a comparator 11, with the base of the transistor Tr1 serving as a non-inverting input terminal, the base of the transistor Tr6 serving as an inverting input terminal, and the node between the collectors of the transistors Tr3 and Tr5 serving as an output terminal. That is, the comparator 11 receives, at its non-inverting input terminal, the voltage VBG through the switch 1 and, at its inverting input terminal, a voltage obtained by dividing the output voltage appearing at the output terminal 6 with the resistors R2 and R3, thereby forming a negative feedback circuit.
In this power supply device, when the switch 2 is closed, the supply voltage Vcc is applied to the constant-current sources 3, 4, and 5, to the resistor R1, and to the emitter of the transistor Tr8. Simultaneously, the switch 1 is switched to its contact xe2x80x9cb,xe2x80x9d so that the input voltage VBG is applied to the base of the transistor Tr1. Making the base voltage of the transistor Tr1 equal to VBG in this way brings the transistor Tr1 into a non-conducting state. This reduces the current flowing from the base of the transistor Tr2 to the emitter of the transistor Tr1, and thus makes the emitter current of the transistor Tr3 larger than the emitter current of the transistor Tr2. On the other hand, the transistors Tr4 and Tr5 together constitute a current mirror circuit, and therefore the collector current s of the transistors Tr4 and Tr5 are equal to the emitter current of the transistor Tr2.
As a result, a current flows from the comparator 11 to the base of the transistor Tr7. This base current causes an amplified current to flow through the transistor Tr7 as its collector current, and the resulting voltage drop across the resistor R1 causes the base voltage of the transistor Tr8 to drop. Thus, an emitter current starts flowing through the transistor Tr8, delivering an output voltage Vo to the output terminal 6.
In this way, the power supply device configured as shown in FIG. 6 outputs the output voltage Vo via its output terminal 6. Here, the output voltage Vo takes several milliseconds to rise, and therefore a start-up charge current (hereinafter referred to as the xe2x80x9cinrush currentxe2x80x9d) as large as 1 A or more flows through the capacitor Co. This inrush current flows to the limit of the current capacity of the output transistors of the power supply device, and therefore, in a case where the output current rises abruptly, as in the conventional power supply device under discussion, the heat accompanying the large inrush current may degrade the characteristics of, or even destroy, the power supply device. Moreover, for example, in a case where the source of the supply voltage Vcc is of a DC/DC type, the rush current causes a drop in the supply voltage Vcc, and is thus likely to cause start-up failure in all circuits that are used in parallel with the power supply device.
An object of the present invention is to provide a power supply device provided with a soft starting function whereby the voltage that is fed in at start-up is increased gradually so that the output voltage rises gradually in order to reduce the inrush current at start-up.
To achieve the above object, according to claim 1, a power supply device that controls an output voltage from an output circuit by comparing a monitoring voltage, which is obtained by dividing the output voltage, with a reference voltage by means of a comparator and keeping the monitoring voltage equal to the reference voltage on the basis of a comparison output from the comparator comprises a soft starting circuit that, at star-up, outputs a gradually increasing voltage and shuts off the reference voltage until the gradually increasing voltage reaches a predetermined voltage higher than the reference voltage.
In this power supply device, at the start-up thereof, the voltage output from the soft starting circuit increases gradually until it reaches the predetermined voltage, and, until this voltage output from the soft starting circuit reaches the redetermined voltage, the reference voltage fed to the comparator is shut off. This makes the voltage fed to the comparator vary gradually, and thereby suppresses the transient response of the output voltage from the output circuit at start-up.